The present invention relates to the field of placing soldering paste into and onto electronic circuit boards, ceramic substrates and laminate packages. More particularly, this invention deals with the periodic process of cleaning a printer bar with paper and an apparatus for carrying out this operation.
A common structure in various electronics packages, such as laminate packages, wired circuit boards, ceramic substrates, and hybrid circuits, is a via. A via is a vertical opening filled with conducting material used to connect circuits on various layers of a device or electronics packages to one another. Vias in certain devices may connect to a semiconducting substrate. A via generally starts as an empty cylindrical opening in an electronics package. During manufacture, the via is initially filled with soldering paste. The part is soldered or placed in a solder bath resulting in the via being filled with solder, which is the conducting material of the via as defined above.
The most common way to insert the soldering paste in a via has been to use a squeegee printing method. The squeegee printing method moves a squeegee over a printed circuit board held in position on a back plate. A screen with openings therein, also known as a stencil, is placed between the printed circuit board and the squeegee. The screen or stencil prevents the solder paste from being applied to areas where it is not required.
During the printing operation, the solder paste is being dispensed through the patterned stencil onto the printed circuit board (xe2x80x9cPCBxe2x80x9d). The top printed surface of the PCB comes into direct contact with the underside of the stencil during the paste printing process. During contact the small gap between the underside of the stencil and the board surface may become filled with excess solder paste. The excess solder may clog the aperture in the stencil. Hence it is compulsory to clean the underside of the stencil surface periodically.
The solder paste printing machine comes with stencil underside cleaning options. Cleaning is done manually or automatically at preset intervals. The cleaning option adopts a wiping system that uses lint free paper together with some cleaning chemical.
Several manufacturers have heads designed for surface mount printing of via filling paste. These heads rely on low pressure and a material rolling effect for filling vias.
Different models of screen printing machines use different types of cleaning paper for this purpose. Different types of screen printing machines come with unique features for its applications. Therefore it is common for any PCB Assembly house to have more than one type of screen printer. However, different types of screen printer use different types of wiping paper. Though all are lint free paper, the dimensions can vary significantly resulting in a large cost variation. This will create a high inventory on wiping paper type, as required to support the operation as necessary.
Carrying many types of cleaning paper inventory results in high inventory costs as well as increased complexity in making sure that the various sizes and types of cleaning paper is stocked within the PCB assembly plant. Additional costs are also incurred since each printing machine head manufacturer may charge a premium for cleaning paper that fits a particular print head on a particular model of printing machine.
Thus, there is a need for a method and apparatus for cleaning heads that uses a single type of cleaning paper for more than one type or model of PCB printing head. This has the advantage of simplifying the inventory process and allowing a PCB assembly house to get volume pricing on cleaning paper. There is also a need for a process and apparatus which can be used to more uniformly clean the heads on PCB printers. By making the process more uniform, the process can be more closely controlled and any process errors that may occur due to cleaning errors can be more quickly isolated and corrected. There is also a need for a process which can be adapted for use on all types of heads for all types of printing machines used within a PCB assembly plant. There is also a need for a device which will lessen the possibility that contaminates will be introduced into the via filling paste.
A cleaning element for a solder paste printing machine includes a shaft with an opening therein. Positioned within the opening is a protrusion element. A spring element is also positioned within the opening. The protrusion element further includes a first portion which fits within the opening of the shaft, and a second portion which extends beyond the opening in the shaft. At least a portion of the spring element is sandwiched between the opening in the shaft and the first portion of the protrusion element. Also included is a cover for covering the shaft. The cover has a slot which allows the second portion of the protrusion element to extend beyond the main body of the shaft. The cleaning element may also include a fastener having a threaded end and an upset end. The opening in the shaft also has at least one threaded opening and the cover has a corresponding opening therein for receiving the upset end of the fastener. The amount of pressure produced by the second portion of the protrusion element can be adjusted by adjusting the distance that the fastener is threaded into the threaded opening in the shaft. The second end of the protrusion element includes at least one beveled edge and may have at least two beveled edges. In some embodiments, the spring element is an elastomeric material the thickness of which can be varied to vary the amount of force produced by the protrusion element. In addition, the spring constant of the spring element can be varied to vary the amount of force produced by the protrusion element. In some embodiments, the protrusion element is made of metal. In addition, the shaft can include a second opening therein with a second protrusion element, and a second spring element positioned within the second opening. The second protrusion element further includes a first portion which fits within the second opening of the shaft, and a second portion which extends beyond the second opening in the shaft. At least a portion of the second spring element is sandwiched between the second opening in the shaft and the first portion of the second metal protrusion element. A second cover for covering the shaft is also provided. The cover has a slot therein to allow the second portion of the second protrusion element to extend beyond the main body of the shaft.
In another embodiment, the shaft of the cleaning element for a solder paste printing machine includes a plurality of openings. The cleaning element has a plurality of protrusion elements positioned within the plurality of openings in the shaft.
A solder paste printing machine includes a print head, a screen, and a screen cleaning element. The screen cleaning element includes a first shaft having unused cleaning paper thereon, and a second shaft having used cleaning paper thereon. Each of the first shaft and the second shaft have an opening therein. Each of the first shaft and the second shaft further include a protrusion element and a spring element positioned within the opening in the shaft. Each protrusion element includes a first portion which fits within the opening of the shaft, and a second portion which extends beyond the circumference of the shaft. At least a portion of the spring element is sandwiched between the opening in the shaft and the first portion of the protrusion element. Also included is a first cover for covering the opening in the first shaft and a second cover for covering the opening in the second shaft, the first cover having a slot therein to allow the second portion of the protrusion element to extend beyond the circumference of the first shaft. The second cover also has a slot therein to allow the second portion of the protrusion element to extend beyond the circumference of the second shaft. The solder paste printing machine also includes an adjustment mechanism associated with each of the first second and the second shaft for adjusting the amount of force produced by the protrusion element associated with each of the first shaft and the second shaft. In some embodiments, the adjustment mechanism associated with each of the first shaft and the second shaft includes a fastener having a threaded end. In this embodiment, the opening in each of the first shaft and the second shaft has at least one threaded opening therein. The amount of pressure produced by the second portion of the protrusion element can be adjusted by adjusting the distance that the fastener is threaded into the threaded opening in the first shaft and the second shaft.
The second end of each of the protrusion elements associated with the first shaft and the second shaft includes at least one beveled edge or may have at least two beveled edges. The spring elements associated with the first shaft and the second shaft include an elastomeric material. In some embodiments, at least one of the first shaft and the second shaft has a plurality of protrusion elements. The number of protrusion elements used is related to the force needed to hold a roll of cleaning paper onto the shaft such that there is limited movement between the shaft and the roll of cleaning paper.
Advantageously, the method and apparatus for cleaning heads described uses a single type of cleaning paper for more than one type or model of PCB printing head and printing screen. Although one single type of cleaning paper might not be able to be used throughout a PCB assembly house, this has the advantage of lessening the amount of inventory of cleaning paper needed to be carried to use in all the PCB printing machines used in one PCB assembly house. This also simplifies the inventory ordering process, as less types of cleaning paper need to be ordered. In addition, a PCB assembly house can obtain volume pricing on cleaning paper since more of one type of paper can be ordered for each PCB printing machine. The process and apparatus can be used to more uniformly clean the screens on PCB printers since there will be less types of cleaning paper used or even a single type of cleaning paper used. By making the process more uniform, the process can be more closely controlled and any process errors that may occur due to cleaning errors can be more quickly isolated and corrected. The apparatus can be adapted for use on all types of heads for all types of printing machines used within a PCB assembly plant. The invention also lessens the sources of contaminates that could potentially be introduced into the printing process.